openmp Approximation of PI hand-crafting the #pragma omp reduction


Example

int i;
int n = 1000000;
double area = 0;
double h = 1.0 / n;

#pragma omp parallel shared(n, h)
{
  double thread_area = 0;                      // Private / local variable

  #pragma omp for
  for (i = 1; i <= n; i++)
  {
    double x = h * (i - 0.5);
    thread_area += (4.0 / (1.0 + x*x));
  }

  #pragma omp atomic                       // Applies the reduction manually
  area += thread_area;                     // All threads aggregate into area
}
double pi = h * area;

The threads are spawned in the #pragma omp parallel. Each thread will have an independent/private thread_area that stores its partial addition. The following loop is distributed among threads using #pragma omp for. In this loop, each thread calculates its own thread_area and after this loop, the code sequentially aggregates the area atomically through